Transient operating conditions in an internal combustion engine generally take the form of a dynamically changing engine load and/or engine speed, and to minimize cycle times and/or voltage dips it is accordingly desirable to supply maximum engine output power during such transient conditions. Engine output power is directly proportional to engine speed and engine output torque, the latter of which may be air limited during transient conditions.
With engines including a fixed-geometry turbocharger, the turbocharger's turbine swallowing capacity decreases with increasing engine speed in part due to the nozzle flow characteristic and increased pressure ratio, and also in part due to a reduction in the apparent nozzle area resulting from higher turbine rotor speed, as is known in the art. The turbocharger turbine area thus appears smaller to the incoming exhaust gases at higher engine speeds, thereby resulting in improved turbocharger response as engine speed increases. In order to maintain an optimally responsive turbocharger and thereby maximize engine output power during transient operating conditions, it is therefore important to maintain a high engine speed and minimize speed dips.
Short of developing an engine capable of producing any amount of instantaneous load that an alternator, pump or other engine-driven accessory may apply, some form of load control is typically desired to optimize system performance during transient operating conditions. Some known engine controllers provide only for the ability to ramp applied engine load at a rate designed for operation within a wide tolerance (e.g., +/−3 sigma) of engine performance. Other known engine controllers provide for engine load reduction only when engine speed has dropped below a target value. Unfortunately, neither of these engine controller types take full advantage of the transient torque capability of most engines.
What is therefore needed is a simple and accurate strategy for determining the instantaneous load capability of a supercharged or turbocharged compression ignition engine. The instantaneous engine load production parameter is preferably easily converted to a current maximum available engine output torque value that may be implemented in an engine-driven accessory control scheme, whereby system transient performance can be dynamically optimized by continuously considering the engine's maximum transient load capability.